1. Field of the Invention
The present invention relates in general to a method and an apparatus for lapping or finishing teeth of gears, and more particularly to improvements in such lapping method and apparatus wherein a pair of gears to be lapped are moved relative to each other so as to finish the faces of the gear teeth over the entire width of the faces.
2. Discussion of the Prior Art
For reducing meshing noises of gears, the teeth of the gears are usually lapped so as to improve surface smoothness. In one known lapping process, the faces of the teeth of a pair of gears are finish-lapped over the entire width such that the gears are rotated in meshing engagement with each other, under a loaded condition, while the gears are moved relative to each other so that a teeth contact area of the two gears is moved in the direction of width of the contacting faces of the gear teeth. An example of such a lapping method is disclosed in laid-open Publication No. 64-51222 (published Feb. 27, 1989) of an unexamined Japanese Patent Application, wherein a plurality of lapping cycles are continuously performed, each lapping cycle including a relative movement of the gears to lap the entire width of the tooth faces of the gears.
In the known lapping arrangement wherein a pair of gears are rotated in meshing engagement with each other, a load torque applied to one of the gears, rotating speeds of the gears and a rate of relative movement of the gears are held at predetermined constant values throughout the lapping operation consisting of two or more cycles. This lapping method permits a constant rate of removal of the material over the entire width of the tooth faces of the gears, but does not assure sufficiently high overall lapping efficiency. Further, the known lapping method tend to suffer from overheating of the gears at the end portions of the tooth faces, and tooth interference between the two gears at the non-lapped portions of the teeth when the lapped gears are installed in place for use.
More specifically, the amount of metal that should be removed from the gear teeth differs at different portions of the tooth faces, depending upon the configuration and application condition of the gears to be lapped. In the conventional method as discussed above, the amount of metal removal is constant over the entire width of the tooth faces. Usually, the metal removal amount or lapping depth is set to a largest value that should be theoretically applicable to only a limited area of the tooth faces. This means an unnecessarily long lapping time, since the other areas of the tooth faces does not require such a large amount of metal removal. Further, if the two gears are moved relative to each other such that the actual teeth contact area is reduced from a nominal value at the opposite ends of the width of the tooth faces, the surface pressure at the opposite end portions of the tooth faces is made excessively high, causing an elevated temperature at those end portions, or the metal removal amount is made excessively large at the end portions. On the other hand, if the metal removal amount is unnecessarily large at the central portion of the tooth faces, the amount of backlash of the gears is accordingly increased since it is generally defined by the central portions of the tooth faces. Therefore, when the gears are installed in place with an optimum amount of backlash, the non-lapped portions of the tooth faces of one of the two gears may interfere with the tips of the teeth of the other gear.